Fluorescent polymers useful in conjunction with optical PH sensors

ABSTRACT

An optical sensor is provided for measuring the pH of a fluid sample. The sensor is formulated using a fluorescent polymer composition comprising a copolymer of a water-dispersable, polyether-containing urethane olefin precursor and a fluorescent monomer species, wherein the ratio of precursor and fluorescent monomer species is such that a predetermined apparent pKa is provided. The sensor is prepared by coating the distal end of an optical waveguide with the fluorescent polymer composition, and curing the copolymer contained in the composition, e.g., by exposure to radiation of a suitable wavelength.

This application is a divisional of U.S. application Ser. No.08/074,749, filed Jun. 10, 1993.

TECHNICAL FIELD

The present invention relates generally to optical sensors for measuringthe pH of a fluid, and more particularly relates to a novel opticalsensor system containing a membrane of a fluorescent hydrophilicurethane copolymer. The invention additionally relates to fluorescentpolymeric compositions for use in an optical pH sensor, and to membraneswhich may be manufactured therefrom. One important application of theinvention involves the precise measurement of pH in the physiologicalrange.

BACKGROUND

Chemical sensors are generally known for use in a wide variety of areassuch as medicine, scientific research, industrial applications and thelike. Fiber optic and electrochemical approaches are generally known foruse in situations where it is desired to detect and/or measure theconcentration of a parameter at a remote location without requiringelectrical communication with the remote location. Structures,properties, functions and operational details of fiber optic chemicalsensors can be found in U.S. Pat. No. 4,577,109 to Hirschreid, U.S. Pat.No. 4,785,814 to Kane, and U.S. Pat. No. 4,842,783 to Blaylock, as wellas Seitz, "Chemical Sensors Based on Fiber Optics," AnalyticalChemistry, Vol. 56, No. 1, January 1984, each of which is incorporatedby reference herein.

Publications such as these generally illustrate that it is known toincorporate a chemical sensor into a fiber optic waveguide, anelectrochemical gas sensor or the like, in a manner such that thechemical sensor will interact with the analyte. This interaction resultsin a change in optical properties, which change is probed and detectedthrough the fiber optic waveguide or the like. These optical propertiesof chemical sensor compositions typically involve changes in colors orin color intensities. In these types of systems, it is possible todetect particularly minute changes in the parameter or parameters beingmonitored in order to thereby provide especially sensitive remotemonitoring capabilities. Chemical sensor compositions that areincorporated at the distal end of fiber optic sensors are oftenconfigured as membranes that are secured at the distal tip end of thewaveguide device or optrode.

Sensors of this general type are useful in monitoring the pH of a fluid,measuring gas concentrations such as oxygen and carbon dioxide, and thelike. Ion concentrations can also be detected, such as potassium,sodium, calcium and metal ions.

A typical fiber optic pH sensor positions the sensor material at agenerally distal location with the assistance of various differentsupport means. Support means must be such as to permit interactionbetween the pH indicator and the substance being subjected tomonitoring, measurement and/or detection. With certain arrangements, itis desirable to incorporate membrane components into these types ofdevices. Such membrane components must possess certain properties inorder to be particularly advantageous. Many membrane materials have someadvantageous properties but also have shortcomings. Generally speaking,the materials must be biocompatible, hemocompatible for use in thebloodstream, selectively permeable to hydrogen ions, and of sufficientstrength to permit maneuvering of the device without concern aboutdamage to the sensor.

It is also desirable to have these membrane materials be photocurable(such that curing is neater, can be done more rapidly, on a smallerscale, and directly on the optical fiber), resistant to shear forces(e.g., as present in a bloodstream), and compatible with differentsubstrates, such that there is a choice of fiber optic materials whichcan be used to fabricate the sensor. It is also preferred, clearly, thata signal of sufficient intensity be produced, such that measurement isas accurate as is reasonably possible. The optical pH sensors which arecurrently available are frequently inadequate with regard to one or moreof the aforementioned criteria.

It is additionally desired that the materials used for the sensormembrane be constructed such that pH values in a relatively wide rangemay be accurately measured. To date, this has not been the case withoptical pH sensors. Rather, indicator compositions of prior art opticalsensors typically display an apparent pKa which is substantially lowerthan is desirable for the accurate and precise measurement of a pH aboveabout 7.0, e.g., as is true for the physiological pH range.

The present invention is addressed to novel fluorescent copolymercompositions which have been found to be particularly suitable for useas membranes and membrane-like components in an optical pH sensor andwhich provide for optical sensors which meet each of the above-mentionedcriteria. That is, optical sensors as provided herein tend to adherewell to different types of substrates, eliminating in some cases theneed to silanize the substrate surface, provide for superior signalintensity, are quite hemocompatible relative to prior art compositions,are rapidly cured with light, and are resistant to shear forces such asthose present in flowing blood. Additionally, the novel polymercompositions are such that their apparent pKa may be raised or loweredat will, enabling measurement of a wide range of pH values in a fluidsample. It is preferred that the apparent pKa of the compositions be inthe range of approximately 6.6 to 8.0, more preferably in the range ofapproximately 7.2 to 7.8, most preferably in the range of approximately7.2 to 7.4, such that pH values in the physiological range may beprecisely determined.

OVERVIEW OF RELATED ART

The following references relate to one or more aspects of the presentinvention:

U.S. Pat. No. 4,785,814 to Kane describes an optical probe useful formeasuring pH and oxygen and blood. The device includes a membraneconstructed of a hydrophilic porous material containing a pH-sensitivedye.

U.S. Pat. No. 4,842,783 to Blaylock describes a fiber optic chemicalsensor which, at the distal end of the optical fiber, is provided with aphotocrosslinked polymeric gel having a dye adsorbed therein.

PCT Publication No. WO88/05533, inventors Klainer et al., describes afiber optic sensing device for measuring a chemical or physiologicalparameter of a body fluid or tissue, in which a polymer containingphotoactive moieties is directly bound to the fiber optic tip.

PCT Publication No. WO90/00572, inventors Boesterling et al., describethe use of a urethane or an acrylamide hydrogel for measuring pH and/orpCO₂ in a fluid. The hydrogels are prepared by reacting an isocyanateprepolymer with a derivatized azo dye, i.e., an absorbance dye which isa molecule containing either an amino or hydroxyl functionality.

H. J. Hageman et al., "Photoinitiators and Photocatalysts for VariousPolymerisation and Crosslinking Processes," in Radiation Curing ofPolymers II, ed. D. R. Randell (The Royal Society of Chemistry, 1991),at pp. 46-53, identify a number of materials which will act to catalyzeradiation curing of multifunctional monomers or oligomers.

SUMMARY OF THE INVENTION

Accordingly, it is a primary object of the invention to address theabove-mentioned needs in the art, by providing an optical sensor formeasuring the pH of a fluid, which sensor gives rise to the numerousadvantages identified above.

It is another object of the invention to address these needs byproviding a fluorescent polymer composition for incorporation into suchan optical sensor, wherein the fluorescent polymer composition comprisesa copolymer of a water-dispersable, polyether-containing urethane olefinprecursor and a copolymerizable monomeric fluorescent indicator species.

It is still another object of the invention to provide a membranefabricated from the aforementioned fluorescent polymer composition.

It is yet another object of the invention to provide a method for makingan optical sensor containing the aforementioned fluorescent compositionby copolymerizing the water-dispersable, polyether-containing urethaneolefin precursor and the copolymerizable monomeric fluorescent indicatorspecies on the fiber optic tip.

It is a further object of the invention to provide such a method whichinvolves irradiating the precursor and the copolymerizable monomericfluorescent indicator species through the fiber.

Additional objects, advantages and novel features of the invention willbe set forth in part in the description which follows, and in part willbecome apparent to those skilled in the art upon examination of thefollowing, or may be learned by practice of the invention.

In one aspect, then, a fluorescent polymer composition is provided whichis useful in an optical sensor for the measurement of pH in a fluidsample. The fluorescent polymer composition comprises a copolymer of (a)a water-dispersable, polyether-containing urethane olefin precursor, and(b) a monomeric fluorescent indicator species. The ratio of the urethaneolefin precursor to the fluorescent species is calculated so as toprovide the composition with a predetermined apparent pKa. Generally, itis preferred that the ratio be such that the apparent pKa of thecomposition is in the range of about 6.6 to 8.0, more preferably in therange of about 7.2 to 7.8, most preferably in the range of about 7.2 to7.4, which in turn optimizes the composition for use in measuring pH inthe physiological range.

In another aspect, a cross-linked, selectively permeable (i.e., H⁺-permeable) membrane is provided which is useful for fabricating anoptical pH sensor. The membrane comprises a polymeric matrix of thenovel fluorescent composition.

In still another aspect, an optical sensor is provided for themeasurement of pH in a fluid sample, comprising an optical waveguide toreceive light from a light source, and a pH-sensitive medium disposed onthe waveguide which fluoresces in response to light from the lightsource, wherein the intensity of fluorescence is dependent on the pH ofthe environment being monitored, and the pH-sensitive medium comprisesthe aforementioned fluorescent polymer composition. The pH-sensitivemedium may or may not be present in the form of a membrane.

Still other aspects of the invention, as noted above, involve methodsfor making and using the novel optical sensors.

BRIEF DESCRIPTION OF THE DRAWING

In the course of this description, reference will be made to theattached drawings, wherein:

FIG. 1 is a graph illustrating the effect of polymer used on pKa, forhydroxyethylmethacrylate- and for the polyurethane-based fluorescentcomposition of the invention.

FIG. 2 is a generally schematic view of a chemical sensor deviceaccording to the present invention as incorporated in a fiber optic pHsensor device; and

FIG. 3 is an enlarged, detail and generally schematic view of the distalend portion of a sensor device generally in accordance with FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

Before the present compositions, membranes, sensors and methods ofmanufacture are disclosed and described, it is to be understood thatthis invention is not limited to specific sensor formats, specificmembrane compositions, or particular curing processes, as such may, ofcourse, vary. It is also to be understood that the terminology usedherein is for the purpose of describing particular embodiments only andis not intended to be limiting.

It must be noted that, as used in the specification and the appendedclaims, the singular forms "a," "an" and "the" include plural referentsunless the context clearly dictates otherwise. Thus, for example,reference to "a water-dispersable, polyether-containing urethane olefinprecursor" includes mixtures of such precursors, reference to "afluorescent indicator species" includes mixtures of two or more suchindicator species, and the like.

In describing and claiming the present invention, the followingterminology will be used in accordance with the definitions set outbelow.

The term "polymer" as used herein is intended to include both oligomericand polymeric materials, i.e., compounds which include two or moremonomeric units. The term is also intended to include "copolymeric"materials, i.e., containing two or more different monomeric units.

The term "urethane" is used herein in its conventional sense to denoteorganic compounds containing a recurring -O-(CO)-NH- linkage. The term"polyether-containing urethane" is intended to mean a polymer containingrecurring urethane units as just defined, as well as recurring etherlinkages [-(CH₂)_(n) -O-] where n is an integer greater than 1,typically in the range of 1 to 6, inclusive, most typically 1, 2 or 3.

The term "olefin" is used in its conventional sense to mean a molecularentity containing a double bond; the preferred olefinic species of theinvention are molecular entities containing at least one terminusrepresented by the general formula -R-(CO)-CR'=CH₂, wherein R is O orNH, and R' is H or lower alkyl. Although such olefinic termini arepreferred, it will be appreciated by those skilled in the art that otherolefinic termini may be substituted therefor.

The term "precursor" is used herein to mean a compound which whenpolymerized, copolymerized and/or cross-linked will give rise to adesired polymer.

The term "water-dispersable" as used herein is intended to meancompatible with water or with aqueous solutions. Typically, water uptakeby cured films of the "water-dispersable" precursor used to form thefluorescent copolymer will be at least about 10% by weight, morepreferably at least about 20% by weight, and most preferably at leastabout 35% by weight.

In describing chemical compounds herein, the term "lower alkyl" is usedin its conventional sense to mean an alkyl group of 1 to 6 carbon atoms,e.g., methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, t-butyl,and the like.

The fluorescent polymer composition which serves as the sensing meansfor the measurement of pH, as noted above, comprises a copolymer of awater-dispersable, polyether-containing urethane olefin precursor and acopolymerizable fluorescent monomer species. The preferred molecularweight of the copolymer is in the range of approximately 1000 to 25,000,more preferably in the range of approximately 1800 to 5000, and mostpreferably in the range of approximately 1800 to 2500. The urethaneolefin precursor contains recurring urethane units, recurring etherlinkages, and olefinic termini represented by the general formula-R-(CO)-CR'=CH₂, wherein R and R' are as defined earlier; it should alsobe noted that the R and R' at the different termini are not necessarilythe same (i.e., R may be O at one terminus and NH at the other, and,similarly, R' may be H at one terminus and CH₃ at another). Preferredurethane olefin precursors are represented by the general structuralformula ##STR1## wherein: R and R' are as defined previously; n istypically in the range of 1 to about 6; Ar is a monocyclic aromaticmoiety, preferably phenyl, either unsubstituted or substituted with oneto four substituents which are selected so as not to interfere withpolymerization or use of the cured polymer in the pH sensor, e.g., loweralkyl, halogen, nitro and the like; and X is a polyether linkagecontaining approximately 2 to 100, preferably 10 to 50, most preferably15 to 25, recurring mer units having the structure [-(CH₂)_(n) -O-]where n is an integer of 1 or more, typically 1 to 6, more typically 2or 3, i.e., (-CH₂ -CH₂ -O-), (-CH₂ -CH₂ -CH₂ -O-), or combinationsthereof. These polyether linkages may be further substituted with anadditional -CO-NH-Ar-NH-CO₂ -(CH₂)_(n) -R-(CO)-CR'=CH₂ group to providea trifunctional urethane olefin precursor. Methods for synthesizingurethane olefin precursors are described in detail in commonly assigned,copending U.S. patent application Ser. No. 07/911,175, entitled"Cross-Linked Gas Permeable Membrane of a Cured Perfluorinated UrethanePolymer, and Optical Gas Sensors Fabricated Therewith," filed 12 Aug.1992, the contents of which are hereby incorporated by reference.Briefly, that method involves conversion of isocyanate-terminatedcompounds represented by the general formula wherein X is as definedabove, to urethane olefin precursors by replacing the terminalisocyanate moieties with olefin termini. The isocyanate-terminatedstarting material is reacted with the desired olefin monomer oroligomer, i.e., a hydroxy- or amine-terminated acrylate, methacrylate,acrylamide, methacrylamide, e.g., hydroxyethylmethacrylate (in whichcase n=2). Urethane precursors are commercially available from W. R.Grace & Co. (Lexington, Mass.), and Miles Laboratories, Inc.(Pittsburgh, Pa.). Urethane olefin precursors such as 9454, 9455, 9734and 9467, and RCC-12-893, Photomers 6230 and 6264 are commerciallyavailable from Monomer-Polymer and Dajec Laboratories, Inc. (Trevose,Pa.) and Henkel Corporation (Ambler, Pa.), respectively.

The copolymerizable fluorescent monomer species may be virtually anyfluorescent dye or material which is sensitive to pH, modified so as toenable copolymerization with the urethane olefin precursor. Exemplarypreferred modifications include incorporation into the fluorescentmonomer species of a reactive moiety such as acrylate, acrylamide, allylesters, allyl amides, and the like, such that the fluorescent species isthereby incorporated into the backbone of the polymer chain. Examples offluorescent monomer species useful herein include fluorescein andfluorescein derivatives such as carboxyfluorescein, fluoresceinacrylamide, fluorescein isothiocyanate, coumarin,seminaphthorhodafluorescein, seminaphthofluorescein, naphthofluorescein,hydroxypyrene trisulfonic acid and dichlorofluorescein, and the like,again, bearing a reactive site effective to promote incorporation intothe copolymer.

An important advantage of the invention is the capability of adjustingthe apparent pKa of the resultant fluorescent polymer composition byvarying the ratio of polymerizable fluorescent indicator species tourethane acrylate precursor in the copolymer. The tuning of the apparentpKa in this way gives rise to an indicator which generates a maximalsignal change in the desired pH range. An example of this is the use offluorescein dyes which have a published pKa of 6.5. By varying the ratioof such fluorescein dye monomers the apparent pKa in the resultantpolymer system is in the range of about 7.2 to 7.8. That is, themaximally emissive form of the dye, the dianion, undergoes ionization inthe physiological range, pH 7.2 to 7.8, rather than a full pH unitlower. The fluorescent polymer then becomes far more fluorescent in thedesired pH range, giving maximum sensitivity in the physiological range.A lower apparent pKa can be achieved in this system by varying theamount and nature of additional comonomers. To achieve a pKa in therange of about 7.2 to 7.8 the ratio of polymerizable fluorescentcomonomer to polyether-containing urethane species, in equivalents,should be in the range of about 0.001 to about 0.100, more preferably inthe range of about 0.005 to about 0.015.

The optical sensors of the invention are typically prepared by firstmaking a coating solution of the water-dispersable urethane olefinprecursor and the copolymerizable monomeric species. The coatingsolution is prepared by admixing the urethane olefin precursor, thephotoinitiator and the fluorescent monomer species, modified asdescribed above, in a suitable solvent. The total amount of thedissolved solids is typically in the range of about 50% to 75% byweight. Generally, the coating solution will contain on the order of 35to 90 g urethane olefin precursor, 1×10⁻³ to 5×10⁻² g photoinitiator,and 5×10⁻⁴ to 5×10⁻² g fluorescent monomer per 100 g solution. (It maybe noted from the aforementioned preferred ranges that the ratio of dyeto polymer, in equivalents, is in the range of about 0.001 to 0.100,more preferably in the range of about 0.005 to about 0.015.) Preferredsolvents include water-miscible, low boiling point solvents such asmethanol and ethanol, and partially water-miscible solvents such asethyl acetate. A particularly preferred low boiling point solvent is 40%ethanol in water. However, water-miscible, polar solvents that havehigher boiling points may also be used, such as dimethylsulfoxide,dimethylformamide, N-methylpyrrolidone, dimethylacetamide, or the like.A preferred high boiling point solvent is dimethylsulfoxide used atabout 25% by weight.

The coating solution is applied to the distal end of an optical fiber bycoating, painting, dipping, or the like, and then cured on the fiber.Alternatively, the copolymer may be cured first and then affixed to thefiber optic tip. However, the former method is preferred. To cross-linkthe urethane olefin precursor, a cross-linking agent may be dissolved inthe coating solution. The cross-linking agent may be any di- ormultifunctional acrylate. Examples of such cross-linking agents include##STR2## wherein R and R' are as defined above, m is 1, 2 or 3, and X'is a polyether substituent as defined above or a hydrocarbon, preferablyalkylene, group containing about 2 to 20 carbon atoms.

Curing may be carried out by exposing the aforementioned solution,preferably in the form of a coating on the fiber optic tip, to radiationof a wavelength effective to initiate copolymerization. In aparticularly preferred embodiment, curing is carried out on the fibersubstrate using radiation transmitted through the fiber, i.e., after asolution containing the urethane olefin precursor and the fluorescentmonomeric species has been provided on the fiber tip by coating,painting, dipping, or the like. Alternatively, as alluded to above, thefluorescent polymer composition may be cured to form a cross-linkedmembrane, which is then deposited on the surface of the optical fiber.With glass fibers, it has typically been necessary to prime the fibersurface prior to photopolymerization or deposition of the sensingmembrane thereonto. An example of a suitable glass primer isγ-methacryloxypropyl trimethoxysilane. Once cured, the sensor thusformed may be cleaned of residual unreacted monomer by soaking in aninnocuous solvent such as dimethylsulfoxide or water.

As an alternative to photocuring, some systems may be curable uponexposure to heat, preferably using temperatures in the range of about40° C. to about 100° C. and a thermal initiator, e.g., a peroxide or avinyl polymerization initiator such as that available under the nameVazo® from E.I. DuPont de Nemours & Co., Wilmington, Del.

When curing is effected using radiation, it is necessary to carry outthe curing step in the presence of a photoinitiator. Suitablephotoinitiators are radical photoinitiators that are well known to thoseskilled in the art. Examples of such photoinitiators include α-alkoxydeoxybenzoins, α,α-dialkoxy deoxybenzoins, α,α-dialkoxy acetophenones,2-hydroxy-2,2-dialkyl acetophenones, benzophenones, thioxanthones,benzils, and other compounds identified by H. J. Hageman et al.,"Photoinitiators and Photocatalysts for Various Polymerisation andCrosslinking Processes," in Radiation Curing of Polymers II, ed. D. R.Randell (The Royal Society of Chemistry, 1991), at pp. 46-53, citedsupra. The disclosure of the aforementioned reference is incorporated byreference herein.

FIGS. 2 and 3 show a typical fiber optic pH sensor arrangement. Theillustrated device 11 includes a light source 12 for directing proberadiation into the device, as well as a light detector 13 for sensingand detecting radiation from the device. Device 11 includes one or moreoptical fibers 14 that are joined to light source 123 and to lightdetector 13 through a suitable junction assembly 15 at a location whichis proximal of the distal end portion 16 of the optical fiber 14. As isgenerally known, each optical fiber 14 includes a core surrounded by acladding or covering.

Distal end portion 16 has a distal tip 17 which is a membrane of acopolymer of a water-dispersable, polyether-containing urethane olefinprecursor and a copolymerizable fluorescent monomer species. Thefluorescent monomer species enables the membrane to undergo a knownchange in color, color intensity or other property, which change isobserved by the light detector 13 in a manner generally known in theart.

Examples of suitable fiber substrate materials include glass, plastic,glass/glass composite and glass/plastic composite fiber waveguides. Acritical characteristic of optical fibers is attenuation of the opticalsignal. Thus, glasses which contain unacceptable levels oftransition-metal impurities when prepared from naturally occurringmaterials lead to high absorption losses. High silica fibers ofacceptable quality can be prepared from purified starting materials(e.g., silicon tetrachloride and germanium tetrachloride) usingconventional glass-melting techniques of melting, fining and drawinginto fibers. In order to promote adhesion of the membrane to the fiber,the surface of the tip of the fiber substrate may be silanized, such aswith γ-methacryloxypropyl trimethoxysilane as primer, as discussedabove.

As noted earlier, the primary utility of the present invention is in thedetection and measurement of pH. However, the membrane and sensor of theinvention may also be used in a variety of other contexts, e.g., indetecting and/or quantitating O₂ and CO₂, for on-line sensing in aflowing fluid stream, or in static application.

It is to be understood that while the invention has been described inconjunction with preferred specific embodiments thereof, the foregoingdescription, as well as the examples which follow, are intended toillustrate and not limit the scope of the invention. Other aspects,advantages and modifications within the scope of the invention will beapparent to those skilled in the art to which the invention pertains.

EXAMPLE 1 Synthesis of Polyurethanemethacrylamide, a HydrophilicUrethane Olefin Prepolymer

Using a clean, dry, metal spatula, 18.39 g (0.0287 equivalents) Hypol®2000 (W. R. Grace & Co., Lexington, Mass.), a hydrophilic prepolymer,was transferred from its storage container, which had been purged withargon during and for two minutes after the transfer, to a 50 mlflame-dried reaction kettle. RecrystallizedN-(2-hydroxypropyl)methacrylamide (HPMA) (4.11 g; 0.0287 equivalents)(Polyscience Corp., Warrington, Pa.) was rapidly added to the reactionkettle and, immediately thereafter, the kettle was purged with argonafter replacing and securely clamping the lid in place. An air-drivenstirrer equipped with a glass shaft and Teflon stir bar was used to mixthe prepolymer while continuing to purge with argon. Using a syringe,redistilled dimethylsulfoxide (7.5 g), which had been stored in a rubberseptum-covered container, was transferred into the reaction kettle. Thereaction mixture was stirred until the HPMA was completely dissolved(about 15 min). Dabco 33-LV catalyst (Air Products & Chemicals, Inc.,Allentown, Pa.) (10 μL) was added to the reaction kettle as a bolus. Thereaction mixture was stirred, as above, at room temperature under argonfor one week. The reaction was complete after this time as determined byIR (disappearance of NCO absorption band at ˜2270 cm⁻¹) and GC (nofurther change in concentration of residual HMPA with time).

EXAMPLE 2 Preparation of pH Sensors

Fluorescein acrylamide, isomer I (10 mg) was dissolved in 250 μL ofabsolute alcohol. One hundred microliters of this solution was added to1.0 g of polyurethanemethacrylamide (PUMAM), synthesized in thepreceding example. Darocur® 1173 (Ciba-Geigy Corp., Hawthorne, N.Y.) aphotoinitiator, (3 μL) and ascorbic acid palmitate (20 mg) were added tothe PUMAM/fluorescein acrylamide solution. The mixture was stirred witha glass rod until homogeneous. The homogeneous mixture was placed in anoven at 50° C. for 18 minutes to dissolve the ascorbic acid palmitate.Silanized fibers (plastic-clad glass, obtained from Ensign-Bickford)were dipped into this mixture and removed prior to curing with a mediumpressure arc lamp (340±15 nm bandpass; Blackray) so that only a thinfilm remained on the tip of the fiber. The resulting sensors were foundto be pH responsive and linear with respect to pH values in the range of6.8 to 7.8.

EXAMPLE 3 Synthesis of a High Molecular Weight, Co-macromonomer ofFluorescein Acrylamide, Isomer I and Preparation of pH Sensors

Using a clean, dry, metal spatula, 10.0 g (0.0625 equivalents) Hypol®2002 hydrophilic prepolymer (W. R. Grace & Co., Lexington, Mass.) wastransferred from its storage container, which had been purged with argonduring and for two minutes after the transfer, to a 50 ml flame-driedreaction kettle. Dried hydroxyethylmethacrylate (HEMA) (2.08 g, 0.0625equivalents) (Polyscience Corp., Warrington, Pa.) was rapidly added tothe reaction kettle and, immediately thereafter, the kettle was purgedwith argon after replacing and securely clamping the lid in place. Astirrer was used to mix the prepolymer while continuing to purge withargon. Ethyl acetate (10.0 g) and dibutyltin dilaurate (5 μL) catalystwere added, and the reaction mixture was incubated at room temperaturefor two hours. The mixture was then dried under a stream of nitrogen anda sample of the mixture cured by exposure to a medium pressure arc lamp.

A monomer mixture was then prepared containing 1.5 g of theabove-prepared prepolymer, 0.5 g ethanol, 0.005 g fluoresceinacrylamide, and 0.099 g Irgacure®-500 (Ciba-Geigy, Hawthorne, N.Y.)photoinitiator. The monomer mixture was grafted onto a silanizedglass/plastic fiber and cured with ultraviolet radiation. The pH sensorthus prepared was found to be pH responsive with respect to measurementof pH values in the range of 6.5 to 8.2.

Similar monomer mixtures were then prepared containing 1.5 g of theprepolymer, 0.5 g ethanol, varying amounts of fluorescein acrylamide asindicated in the graph of FIG. 1, and ethylene glycol dimethacrylate(EGDMA) (approximately 0.5 wt. % relative to the prepolymer), tocrosslink the polymer. The mixtures were grafted onto silanizedglass/plastic fibers as above, and cured with ultraviolet radiation. Theeffect of dye/polymer ratios on pKa was then evaluated and plotted inFIG. 1.

EXAMPLE 4 Synthesis of Polyurethane Polyethylene Glycol a HydrophilicUrethane Olefin Prepolymer

Using a clean, dry, metal spatula, 11.72 g (0.0183 equivalents) Hypol®2000 hydrophilic prepolymer (W. R. Grace & Co.) was transferred from itsstorage container, which had been purged with argon during and for twominutes after the transfer, to a 50 ml flame-dried reaction kettle.Hydroxy-terminated polyethylene glycol 1000 methacrylate (18.28 g;0.0183 equivalents) (Monomer-Polymer and Dajec Laboratories ["MTM"],Trevose, Pa.) was rapidly added to the reaction kettle and, immediatelythereafter, the kettle was purged with argon after replacing andsecurely clamping the lid. An air stirrer was used to mix the prepolymerwhile continuing to purge with argon. Using a syringe, distilleddimethylsulfoxide (10.0 g), which had been stored in a rubberseptum-covered container, was transferred into the reaction kettle. Thereaction mixture was stirred about 15 minutes. Dabco 33-LV catalyst (AirProducts & Chemicals, Inc., Allentown, Pa.) (10 μL) was added to thereaction kettle as a bolus. The reaction mixture was stirred, as above,at room temperature under argon for two days. The reaction was completeafter this time as determined by IR (disappearance of NCO absorptionband at ˜2270 cm⁻¹). The resulting prepolymer can be used to prepare pHsensors as described in Example 3.

We claim:
 1. A fluorescent polymer composition useful in an opticalsensor for the measurement of pH in a fluid sample, comprising acopolymer of (a) a water-dispersable urethane olefin precursorcontaining recurring ether linkages, and (b) a copolymerizable monomericfluorescent indicator species, wherein the ratio of fluorescentindicator species to urethane olefin precursor in the copolymer isselected to provide the copolymer with an apparent pKa of at least about6.6 to about 8.0.
 2. The fluorescent polymeric composition of claim 1,wherein the ratio of fluorescent indicator species to urethane olefinprecursor in the copolymer is selected to provide the copolymer with anapparent pKa in the range of about 7.2 to about 7.8.
 3. The fluorescentpolymeric composition of claim 1, wherein the ratio of fluorescentindicator species to urethane olefin precursor in the copolymer, inequivalents, is in the range of about 0.001 to about 0.100.
 4. Thefluorescent polymeric composition of claim 3, wherein the ratio offluorescent indicator species to urethane olefin precursor in thecopolymer, in equivalents, is in the range of about 0.005 to about0.015.
 5. The fluorescent polymeric composition of claim 1, wherein theurethane olefin precursor contains recurring urethane units -O-(CO)-NH-,recurring polyether units [-CH₂)_(n) -O-] where n is an integer in therange of 1 to 6, inclusive, and termini of the general formula-R-(CO)-CR'=CH₂, wherein R is O or NH, and R' is H or lower alkyl. 6.The fluorescent polymeric composition of claim 5, wherein the urethaneolefin precursor has the structural formula ##STR3## in which: R is O orNH;R' is H or lower alkyl; Ar is a monocyclic aromatic moiety, eitherunsubstituted or substituted with one to four substituents selected soas not to interfere with polymerization or use of the cured polymer inthe pH sensor; X is a polyether linkage containing approximately 2 to100 recurring mer units having the structure [-(CH₂)_(n) -O-]; and n isan integer in the range of 1 to 6 inclusive.
 7. The fluorescentpolymeric composition of claim 6, wherein n is 2 or
 3. 8. Thefluorescent polymeric composition of claim 6, wherein the urethaneolefin precursor has a molecular weight in the range of approximately1,000 to 25,000.
 9. The fluorescent polymeric composition of claim 8,wherein the urethane olefin precursor has a molecular weight in therange of approximately 1,800 to approximately 5,000.
 10. The fluorescentpolymeric composition of claim 9, wherein the urethane olefin precursorhas a molecular weight in the range of approximately 1,800 toapproximately 2,500.
 11. The fluorescent polymeric composition of claim1, wherein the fluorescent indicator species is selected from the groupconsisting of fluorescein, carboxyfluorescein, fluorescein acrylamide,fluorescein isothiocyanate, coumarin, seminaphthorhodafluorescein,seminaphthofluorescein, naphthofluorescein, hydroxypyrene trisulfonicacid and dichlorofluorescein, said species modified so as to enablecopolymerization with the urethane olefin precursor.
 12. The fluorescentpolymeric composition of claim 11, wherein the modification comprisesincorporation into the fluorescent indicator species of a more reactivemoiety selected from the group consisting of acrylate, acrylamide, allylesters and allyl amides.
 13. A fluorescent polymeric composition usefulin an optical sensor for the measurement of pH in a fluid sample,comprising a copolymer of (a) a water-dispersable urethane olefinprecursor containing recurring ether linkages and having the structuralformula ##STR4## wherein R is O or NH, R' is H or lower alkyl, Ar is amonocyclic aromatic moiety, either unsubstituted or substituted with oneto four substituents selected so as not to interfere with polymerizationor use of the cured polymer in the pH sensor, X is a polyether linkagecontaining approximately 2 to 100 recurring mer units having thestructure [-(CH₂)_(n) -O-], and n is an integer in the range of 1 to 6,inclusive, and (b) a copolymerizable, monomeric fluorescent indicatorspecies selected from the group consisting of fluorescein,carboxyfluorescein, fluorescein acrylamide, fluorescein isothiocyanate,coumarin, seminaphthorhodafluorescein, seminaphthofluorescein,naphthofluorescein, hydroxypyrene trisulfonic acid anddichlorofluorescein, said species modified so as to enablecopolymerization with the urethane olefin precursor, wherein the ratioof fluorescent indicator species to urethane olefin precursor, inequivalents, is in the range of about 0.001 to about 0.100.